Field of the Invention
The present disclosure relates to a sheet processing apparatus for processing sheets.
Description of the Related Art
A technique for the folding of a stack of sheets using folding rollers, by pushing the stack of sheets into a nip between the folding rollers with a thrusting member is known (Japanese Patent Laid-Open No. 2011-241021).
A technique for the pressing of a folding portion of the sheet in a linear manner in advance so as to prevent the problem of the cracking of a back portion on an outer side of a folded sheet (hereinafter, referred to as a back crack), from occurring is also known (Japanese Patent Laid-Open No. 2014-227236).
There is a case in which, when a stack of sheets is pushed into a nip portion between folding rollers with a thrusting member, that only a sheet (a cover sheet) among the stack of sheets that is in contact with the folding rollers is conveyed by the folding rollers causing the sheet to be torn (hereinafter referred to as a tear of the cover sheet). It is desirable that a moving velocity (hereinafter referred to as a thrust speed) of the thrusting member is high in order to prevent the tearing of the cover sheet from occurring. However, when the thrust speed is high, a mark (a thrust plate mark) on the sheet, caused by the action of the thrusting member, may occur when the position where the crease is formed and the thrusting position deviate from each other.